Automatic rain sensors for controlling the windshield wipers on vehicles have employed optical sensors using infrared emitters and detectors to determine whether moisture is present on the windshield surface. Optical rain sensors can be adapted to any windshield without the need for special glass processing.
One such optical sensor is disclosed in U.S. Ser. No. 073,496 filed July 15, 1987, now U.S. Pat. No. 4,798,956, in the name of the inventor named herein. The sensor assembly is adapted to a window having inside and outside surfaces defining a thickness T and of a material having an index of refraction n. The assembly utilizes an emitter for emitting radiant energy toward the window at an incident angle Z relative to the inside surface and a detector for detecting radiant energy from the window at a reflection angle relative to the inside surface and equal to the incident angle Z. The emitter and detector are spaced a distance apart D on the inside surface of the window to compensate for refraction at the unmatched index plane. The distance D is determined by the following equation: EQU D=2 T cosZ (n.sup.2 -cos.sup.2 Z).sup.-1/2
The detector includes a collimator and infrared filter to eliminate the effects of extraneous radiant energy.
This assembly does not compensate for reduction in infrared emission from the emitter LED with increasing ambient temperature nor the partial reduction in photosensor sensitivity by high ambient light levels causing erroneous operation. Careful filtering of the photosensor cell to exclude all wavelengths but those emitted by the pulsed LED IR source does limit the sensitivity of the system to sunlight, however, certain lighting conditions which may cause a sun reflection or glint from car or road surfaces can still induce a detector overload resulting in an erroneous wipe signal.
Use of a reference channel has been utilized for sensing change in emitter output due to temperature or age. One such system is disclosed is U.S. Pat. No. 4,355,271 issued Oct. 19, 1982 in the name of Noack. The system includes an emitter transmitting radiant energy toward the window and reflected to a detector for sensing moisture or particles on the window. A second detector senses the radiant output only of the emitter. The signal from the first detector is compared to the signal from the second detector to determine actuation of the wipers.
A problem with this type of system is that ambient light is not accounted for which can interfere giving inaccurate results. Incident sunlight falling on the photodetector would easily saturate the sensor rendering the assembly useless.